The present invention relates to the field of Magnetic Resonance Imaging (MRI). More specifically, the invention relates to the field of pre saturation based MRI.
Magnetic Resonance Imaging is a useful imaging tool for non-invasive imaging of internal organs. One of the important reasons for the rise in popularity of MRI is the outstanding clarity of the MRI images. Also, MRI uses radio frequency waves rather than X-rays.
An MRI system works on the principle of magnetic resonance to obtain images of the human body. Human body tissues are composed of molecules like water and fat, which contain hydrogen atoms. The nuclei of hydrogen atoms have an associated magnetic moment, which is a measure of the net magnetic properties of the hydrogen nuclei. In a typical MRI system, electromagnets provide a strong magnetic field called the B.sub.0 (B0) magnetic field. The B0 magnetic field interacts with the magnetic moments of hydrogen nuclei and causes a fraction of the nuclei to align with the B0 magnetic field. The B0 magnetic field also causes the magnetic moments of the aligned nuclei to precess around B0 magnetic field direction at a frequency called frequency of precession (ω). The frequency of precession depends on strength of the B0 magnetic field and the gyromagnetic ratio of the nucleus (γ), which is a characteristic property of the nucleus. Mathematically, ω, B0 and γ are related as:ω=γ*B0  equation (1)
The frequency of precession of the nucleus is called the Larmor frequency. The MRI system then applies radio frequency (RF) pulses at the Larmor frequency of hydrogen nuclei. The frequency of the applied RF pulses is called the transmit frequency. When the precessing hydrogen nuclei are exposed to the RF pulses at their Larmor frequency, magnetic resonance occurs. When magnetic resonance occurs, the precessing hydrogen nuclei emit energy in the form of specific radio frequency signals. The radio frequency signals generated by the resonating hydrogen nuclei in water molecules (present in the scan volume) are processed by the MRI system to generate an MRI image. In many applications, the radio frequency signals generated by the resonating hydrogen nuclei in molecules other than water are undesired because they may reduce the quality of the MRI image.
An MRI system images a volume of the human body called the scan volume. One of the ways to image a scan volume is by dividing the scan volume into a number of slices called scan slices. The scan slices are then imaged one by one. The images of these scan slices can then be combined to form an image of the scan volume. To successfully image a scan volume, the B0 magnetic field must be homogeneous across the scan volume. B0 magnetic field inhomogeneity across the scan volume can have an adverse impact on the quality of MRI images. Referring to equation (1), the transmit frequency (which is equal to the Larmor frequency) depends on the strength of the B0 magnetic field. If the B0 magnetic field is inhomogeneous, hydrogen nuclei at different locations of the scan volume will experience different B0 magnetic field strengths. This implies that the transmit frequency to be applied for magnetic resonance to occur will vary across the scan volume. A typical MRI system uses a single transmit frequency for the entire scan volume. Due to this, there will be some nuclei in the scan volume that will not undergo magnetic resonance. These nuclei will not produce a proper radio frequency signal, which leads to a poor quality MRI image. Hence, to obtain a good quality MRI image, a homogeneous B0 magnetic field across the scan volume is desirable.
A homogeneous B0 magnetic field is especially desirable in MRI systems that use pre-saturation. Pre-saturation is a method used for suppressing undesired signals that reduce image quality. In pre-saturation, frequency selective saturation pulses called RF pre-pulses are applied before the RF pulses. The use of frequency selective RF pre-pulses suppresses the undesired signals. An example of such RF pre-pulses is fat saturation. Fat saturation is a technique that selectively suppresses undesired signals from hydrogen nuclei present in fat molecules. Selective suppression is achieved by applying specific fat saturation RF pre-pulses prior to the RF pulses.
Any inhomogeneity in the B0 magnetic field can lead to wrong determination of frequency of RF pre-pulses with respect to the RF pulse used to excite hydrogen nuclei present in water molecules. The use of RF pre-pulses at a wrong frequency substantially affects the image quality. For example in fat saturation, the RF pre-pulses at a wrong frequency will not completely suppress the signals from hydrogen nuclei present in fat molecules. In some cases, the fat saturation pulses of a wrong frequency may suppress signals from hydrogen nuclei present in water molecules, thereby reducing the image quality. FIG. 1 shows the effect of B0 magnetic field inhomogeneity on an MRI image. The MRI image shows poor fat saturation and unwanted suppression of signals from hydrogen nuclei present in water molecules.
Hence, there exists a need for a method to reduce the effect of B0 magnetic field inhomogeneity in pre-saturation based MRI experiments.